The Subterranean World of Gopher Tortoises

THE SLOW-FOOTED GOPHER TORTOISE may not be the most charismatic species, but its burrows are ecological hot spots. Dug deep into the sandy soil of longleaf pine forests and ancient scrub habitats in Florida and adjoining states, these holes attract and support an inordinate number of life-forms—from various snakes, lizards and mammals to several insect species that have never been seen outside a burrow. While scientists still know little about what goes on in these subterranean worlds, one new study suggests the burrows may be more complex than previously thought—a finding supporting suspicions that the humble gopher tortoise is a bona fide ecosystem engineer.

Wildlife's Landlord

All burrows can provide protection from weather and safety from predators. But because the reptile’s tunnels can be nearly 70 feet long and more than 20 feet deep, and often are found in fire-prone habitats, they may be particularly attractive to many creatures. What’s more, gopher tortoises have been digging into Florida soils for two million years—more than enough time for a diverse burrow-centered ecosystem to emerge.

Explorations into this hidden biome date back to 1894, when a Florida entomologist excavated a burrow and found a trove of life-forms. Since then, the number of animal species collected from active or abandoned tortoise burrows has grown to about 400, and evidence suggests some of the creatures would suffer greatly if the tunnels vanished. In one study in Georgia, for example, scientists using radiotelemetry discovered that eastern indigo snakes spend the majority of their time underground, almost always inside tortoise burrows. Such findings aside, attempts to learn more about burrow ecology have been limited by inaccessibility.

Tortoise Tunnels in 3D

Enter former University of Florida graduate student Al Kinlaw. Using a new 3D-imaging system developed by University of Miami researcher Mark Grasmueck—which can record details of objects and voids hidden belowground—Kinlaw revealed for the first time the full shape and extent of the gopher tortoise’s secretive domain. The results, published last year in the journal Geomorphology, show burrows are corkscrew-shaped structures surrounded by cavities that appear to be remnants of former tunnels. The biggest surprise was the extent of the modifications made by secondary residents. One burrow was interlaced with seven smaller tunnels, which Kinlaw suspects were built by another burrow-dependent species, the rare Florida mouse.

Kinlaw believes this “burrowing cascade,” whereby smaller and smaller species extend the tortoise’s handiwork, may help explain the exceptional diversity found in a burrow. Meanwhile, the persistence of tunnel fragments shows how digging can have a lasting impact on the landscape, one of the hallmarks of an ecosystem engineer. “We don’t know how long these relic chambers have been around,” says Kinlaw. “But presumably they provide opportunities for other species.”

Gopher tortoise populations have been reduced by an estimated 70 percent since the first European colonists came to North America, primarily due to habitat loss. Knowing more about the species’ burrows will help conservationists stave off further declines, both of the tortoises and their dependents. “We have some general ideas about what we think happens underground,” says Auburn University herpetologist Craig Guyer. “This technology will allow us to have a solid footing on that.”